The present invention relates generally to a photoelectric position-measuring arrangement and more particularly to a photoelectric position-measuring arrangement which includes a plurality of shiftable grids to diffract light from a light source and bring the diffracted partial-beams into interference such that intensity modulations arising through the interference of the partial-beams are transduced by photo-detectors into electrically signals.
Three-grid measuring arrangements are known in the art. For example, DE-C2-24 31 551 discloses a three-grid arrangement, which includes three transparent grids or gratings which are successively penetrated by light incident thereon. This reference further discloses a second arrangement wherein the system consist of only two grids, one which is transparent and the second which is constructed as a reflection grid. In the systems described in this reference, the grid constant of the grids used cannot be smaller than the wavelength of the light used. Therefore, the resolution of the measuring system is limited.
Another three-grid measuring system is disclosed in EP-A1-163 362. The phase grids disclosed in this reference are configured, with respect to the phase stroke and scanning ratio (strip/furrow ratio), such that three light beams result which undergo intensity modulations and which are phase displaced relative to one another (preferably by 120.degree.). The grid constant in this arrangement also cannot be smaller than the wavelength of the incident light.
DE-A1-21 46 310 discloses the so-called two-grid measuring system. In this two-grid measuring system, the grid constants of the grids used are smaller than the wavelength of the incoming light. A theoretical disadvantage, however, of such interferometric two-grid measuring system is that they are sensitive to fluctuations of the grid spacings and are dependent on the spatial coherence of the light which is used.
Therefore, it is a primary object of the present invention to provide a position-measuring arrangement which effectively uses three grids which are interferometrically scanned and which also permits the use of grid constants that are smaller than the wavelength of the light used.
It is a further object of the present invention to provide a position-measuring arrangement which effectively uses three grids wherein the scanning signals are free of harmonics.